1. Field of the Invention
This invention relates to electronic imaging devices, and in particular to a solid state color imager, constituent color-selective photoreceptors, and related methods of manufacture.
2. Related Art
Solid-state imagers (simply referred to below as xe2x80x9cimagersxe2x80x9d) find use in a broad range of applications in many distinct fields of technology including the consumer, industrial, medical, defense and scientific fields. Imagers use an array of photodetectors to convert photons bearing image information into an electrical signal indicative of the image. While certain types of imagers may provide only black and white imaging, color imagers are also available.
Conventional color imagers incorporate color filters to control the spectrum of light incident on photodetectors. Thus, for example, FIG. 1 shows a photodiode array 100 of a color imager that includes four identical photodiodes 102, 104, 106, 108. Each photodiode includes a transistor access structure (e.g., the structure 110) that allows a control circuit to precharge its connected photodiode and readout charge from its connected photodiode. The pixel array 100 is configured in a Bayer pattern (i.e., it includes a red, blue, and two green photodetectors arranged in a rectangular pattern). A selected color filter (either red, green, or blue) is disposed above each photodiode in order to allow only light of that color to strike the photodiode. Thus, in FIG. 1, photodiode 102 is responsive to red light, photodiode 104 is responsive to blue light, and photodiodes 106, 108 are responsive to green light because the appropriate type of filter is disposed above each photodetector. The color filters are typically made of photoresist containing dye or pigment.
Convention color filters, however, suffer from numerous drawbacks. For example, the color filters fade with exposure to high levels of heat and light. When the filters fade, the color response of the color detector changes, often undesirably. In addition, color filter aperture effects reduce the amount of light incident on the color filter that makes its way to the photodetector. In some circumstances, the color detector may experience as much as a 50% loss of light response at the edge of the photodetector array when a wide angle lens is employed.
Furthermore, the color filters attenuate incident light by as much as 50%. The attenuation results in degraded color imager performance at low light levels (i.e., the color imager does not function adequately in low light environments). An additional drawback of color filters arises from cross-talk effects. Because the depth of light absorption in silicon is not uniform across the visible spectrum, longer wavelength light (e.g., 600 nm red light) is absorbed deeper in the silicon (as opposed to blue light, for example, which is absorbed closer to the surface). Free carriers that result from photon absorption can diffuse significant distances before recombination or absorption by a photodetector. The diffusion of free electrons from red light absorption is the most severe and can result in more than 50% of the red light response occurring in green and blue photodiodes that are near a red filter.
Thus, a need exists for an improved color imager that addresses the problems noted above and other previously experienced.
One exemplary implementation of the invention is a photoreceptor with preferential response to a predetermined color of light. The photoreceptor includes a color-selective semiconductor region limited in depth according to a preselected color fractional absorption ratio for the predetermined color of light, and a shallow semiconductor region disposed above the color-selective semiconductor region for absorbing light of wavelengths shorter than the predetermined color of light. As an example, the color-selective semiconductor region may form a first portion of a first photodiode, and the shallow semiconductor region may form a first portion of a second photodiode. Additionally, a deep semiconductor region may be provided below the color-selective semiconductor region for absorbing light of wavelengths longer than the predetermined color. In particular, the photoreceptor may provide preferential response to wavelengths of green light.
Another aspect of the invention is a solid state color imager (xe2x80x9cimagerxe2x80x9d) with preferential response to two or more colors of light. The imager includes a first color-selective photoreceptor built in part from a first color-selective semiconductor region limited in depth according to a first color fractional absorption ratio for a first color of light, as well as a second color-selective photoreceptor built in part from a second color-selective semiconductor region limited in depth according to a second color fractional absorption ratio for a second color of light.
Optionally, the imager may provide a light shield (e.g., metalization or another opaque structure) above the second color-selective semiconductor region and position the second color-selective photoreceptor in proximity to the first color-selective photoreceptor. The second color-selective photoreceptor may then collect electrons diffusing from the first color-selective photoreceptor and generated by the second color of light. Where the imager is intended for applications related to human color vision, the first and second colors of light may be, for example, blue and green, blue and red, and/or green and red wavelengths of light. In machine vision applications, the colors of light may correspond to other predetermined wavelengths.
The imager may include additional color-selective photoreceptors selectively responsive to different colors of light. Thus, for example, the imager may further include a third color-selective photoreceptor located in proximity to the first color-selective photoreceptor and the second color-selective photoreceptor. In an imager in which the third color-selective photoreceptor preferentially responds to green wavelengths of light, for example, the third color-selective photoreceptor may include a third color-selective semiconductor region limited in depth according to a preselected color fractional absorption ratio for green light, and either a shallow semiconductor region disposed above the third color-selective semiconductor region for absorbing light of wavelengths shorter than green light and a deep semiconductor region disposed below the third color-selective semiconductor region for absorbing light of wavelengths longer than green light light.
Generally, the first color-selective photoreceptor is one of many first-color selective photoreceptors, the second color-selective photoreceptor is one of many second-color selective photoreceptors, and the third color-selective photoreceptor is one of many third-color selective photoreceptors arranged on a common substrate to form the imager.
The invention also provides a method manufacturing a solid state color imager. First, a first color-selective photoreceptor is fabricated using a first color-selective semiconductor region limited in depth according to a first color fractional absorption ratio for a first color of light. A second color-selective photoreceptor is fabricated in proximity to the first color-selective photoreceptor using a second color-selective semiconductor region limited in depth according to a second color fractional absorption ratio for a second color of light. Optionally, a light shield may be fabricated above the second color-selective semiconductor region, such that the second color-selective photoreceptor collects electrons diffusing from the first color-selective photoreceptor and generated by the second color of light.
In certain implementations, a shallow semiconductor region may be fabricated above the first color-selective semiconductor region for absorbing light with wavelengths shorter than the first color of light, while a deep semiconductor region may be fabricated below the first color-selective semiconductor region for absorbing light of wavelengths longer than the first color of light. Furthermore, additional color-selective photoreceptors for other colors of light may be created, for example, green-selective, red-selective, and blue-selective photoreceptors on a common substrate to form a color imager.